Monochromators are used to selectively filter an electromagnetic radiation beam so that only a single wavelength or a narrow band of wavelengths is passed by the monochromator. A common type of monochromator utilizes two diffractors, with the input beam diffracted by both to form an output beam which is parallel to the input beam but substantially restricted to a single wavelength. In the visible light range, where the wavelengths are relatively long, diffraction gratings may be used as diffractors. Monochromators which cover a shorter wavelength range, such as in the X-ray region, may use diffractor elements which are formed of single crystals, commonly of silicon or germanium, or multi-laminar layered mirrors that behave in the same manner as crystals.
In an article by J. A. Golovchenko, et al. entitled "X-ray Monochromator System for Use with Synchrotron Radiation Sources," Review of Scientific Instruments, Vol. 52, No. 4, April, 1981, pp. 509-516, a double crystal monochromator is described in which each crystal is oriented to obtain a constant energy or direction as well as a constant beam position as the selected beam energy is varied, and to make the central ray of the beam impinge at the same point on each monochromator crystal independently of the chosen energy. This is achieved in part by sampling the output beam intensity for angular drifts out of parallelism between the two crystals, with an electronic drive then used to correct orientation of the beam. The crystals are mounted on a mounting element composed of two bars joined at a right angle in an "L" shape, with one crystal mounted parallel to one bar and the other mounted perpendicular to the other bar, so that the two crystals are held parallel to one another. The L-shaped mounting member rotates with one of the crystals about the same axis of rotation, to change the wavelength passed by the crystal, while the joint between the two legs of the L and the second crystal both translate along a straight line parallel to the input and output beams. Precision sliders are required between the mounts for the crystals and the mounting bars, as well as at the point at which the center of the L shaped mounting member is supported for translation.
An improved monochromator mounting mechanism is shown in the U.S. Pat. No. 5,157,702 to Middleton and Hicks, issued Oct. 20, 1992, entitled "A Mechanically Actuated Double Crystal Monochromator." The monochromator shown in that patent is designed for operation in high vacuum so that it can be mounted to a synchrotron, and includes precision mechanical parts for allowing the crystal mounting assemblies to slide in precise relation to one another.
In the foregoing systems, the parallelism of the faces of the diffracting crystals is maintained by mounting them on the L-shaped mounting member which is mounted on rotating sliders. Highly precise sliders are required in the structure since the orientation of the crystal is dependent upon the precision of the sliders, which contributes to the complexity and cost of the monochromator instrument.